Eruption of Teeth

Introduction

• Definition: Process by which developing teeth move through jaw bones and overlying mucosa to emerge in the oral cavity.
• Humans as Diphyodonts: Two sets of teeth in a lifetime.
  • Primary teeth (deciduous/baby teeth) start at ~6 months.
  • Permanent teeth replace primary teeth at ~6 years and last a lifetime.

Phases of Tooth Eruption

1. Pre-Eruptive Phase
2. Eruptive (Pre-Functional) Phase
3. Post-Eruptive (Functional) Phase

Pre-Eruptive Phase

• Tooth Germ Growth: Occurs within jaw bones.
  • Jaw bones grow in length, width, and height.
  • Anterior tooth germs move mesially (forward).
  • Molar tooth germs move distally (backward).
  • Jaw bone widening moves tooth germs buckally (outward).
  • Mandibular tooth germs move upwards.
  • Maxillary tooth germs move downwards.
• Types of Movement: Bodily Movements & Eccentric Growth
  • Bodily Movements: Remodeling of bony crypt walls via resorption and deposition.
  • Eccentric Growth: Differential growth of tooth parts aids crown reorientation.
• Position Changes:
  • Permanent tooth germs (successional) start lingual to primary.
  • End up lingual and apical to primary anterior teeth.
  • Permanent premolars rest under primary molar roots.

Eruptive Phase (Pre-Functional)

• Root Formation: Begins simultaneously with this phase.
• Tooth Movement: Ascends through bony crypt and connective tissue.
• Oral Epithelium Contact: Tooth pierces the epithelium forming functional (attachment) epithelium.
• Clearing Pathway: Eruptive path cleared via bone and connective tissue resorption.
  • Factors: CSF1, TGF-beta-1 attract monocytes, differentiate into osteoclasts.
  • Enamel Epithelium: Secretes enzymes that break connective tissue.
• Root Sheath Proliferation: Forms root while collagen bundles form PDL.
  • PDL Formation: Initial fibers start at cervical area and radiate coronally.
  • Involves myofibroblasts with contractile properties.
• Bone Changes: Space below teeth filled with bony trabeculae, providing support, later resorbed for root development.

Post-Eruptive Phase

• Maintaining Position: Ensures teeth stay optimal.
  • Check and correct jaw growth abnormalities.
  • New Bone Deposition: Along socket floor maintains height.
  • Occlusal Wear: Compensated by cemental deposition around apex.
  • Interproximal Wear: Compensated by mesial shift (multiple forces).
• Duration: Continues as long as the tooth remains in the oral cavity.

Timing of Teeth Eruption & Exfoliation

Primary Teeth Eruption (Teething)

• Mandibular Central Incisors: 6-10 months.
• Maxillary Central Incisors: 8-12 months.
• Upper Lateral Incisors: 9-13 months.
• Lower Lateral Incisors: 10-16 months.
• Upper First Molar: 13-19 months.
• Lower First Molar: 14-18 months.
• Upper Canine: 16-22 months.
• Lower Canine: 17-23 months.
• Lower Second Molar: 23-31 months.
• Upper Second Molar: 25-33 months.

Primary Teeth Exfoliation

• Central Incisors: 6-7 years.
• Lateral Incisors: 7-8 years.
• First Molars: 9-11 years.
• Canines and Second Molars: 10-12 years.

Permanent Teeth Eruption

• First Permanent Molars: 6-7 years, no primary exfoliation needed.
• Lower Central Incisors: 6-7 years.
• Upper Central and Lateral Incisors: 7-8 years.
• Lower Canines: 9-10 years.
• First and Second Premolars: 10-12 years.
• Upper Canines: 11-12 years.
• Second Permanent Molars: 12-13 years.
• Third Molars (Wisdom Teeth): 17-25 years or no eruption.

Summary

• Process of Tooth Eruption: Three phases—pre-eruptive, eruptive, post-eruptive.
• Primary Teeth: Start erupting at 6 months, replaced by 6 years.
• Permanent Teeth: Replace primary teeth by processes ensuring functional occlusion and adaptive growth as the jaw develops.
• **Overall: **A dynamic, well-orchestrated sequence ensuring dental health over a lifetime with proper care.